It is an object of the present invention to provide a cabinet capable of containing various devices and securely attaching/detaching an additional device thereto/therefrom, and to provide an additional device. In order to achieve the object, a cabinet according to the present invention includes: a conductive cylinder having an aperture into which the additional device is fitted by insertion and containing an electronic device, the aperture having a folded edge, the additional device operating in parallel with the electronic device; and a reinforcing member supported with a portion of an inner wall of the cylinder and disposed on a boundary between two areas inside the cylinder where the electronic device and the additional device are disposed respectively, the portion of the inner wall being more inside than the folded portion of the aperture.